As one of major components of a mobile communication terminal, such as a cellular phone, is a power amplifier. As a semiconductor device for a power amplifier, a heterojunction bipolar transistor (HBT), which exhibits high efficiency, high linearity, and high current drive capability, is typically used.
Mobile communication terminals are used mainly for consumers, and a reduction in the cost of HBTs is highly demanded. That is, it is desirable to manufacture HBTs by using an inexpensive GaAs substrate instead of an expensive substrate, such as an InP substrate. Thus, developing of high-performance HBTs using a GaAs substrate is highly demanded.
For example, Japanese Unexamined Patent Application Publication No. 2004-71669 discloses a technology for enhancing the performance of HBTs, and more specifically, a technology for reducing the base resistance and the offset voltage (voltage VCE at which the collector current IC starts to flow).